The invention pertains to the field of switching by operator discretion and touch, such as by pressing a key on an electric typewriter or other office machine. In an office machine, such as an electric typewriter, where there is no mechanical linkage between the key and the ultimate event sought by pressing the key (the striking of the raised print-face against the typewriter ribbon and paper), there is no feel in the key movement that signals the completion of the event. The operator must resort to sound or to visual inspection to be sure that the desired result has been achieved. The lack of tactile feel tends to cause excessive pressure applied to the key, decreases speed of operation and decreases the psychological fulfillment of the operator. Consequently, there is a need to provide artificial tactile feel so the operator knows immediately by feel, rather than by resort to sound and/or visual inspection, that there has been sufficient key movement to achieve or complete the desired event.
In prior art devices the requisite breakaway tactile feel has been provided by means of over-center action and by separation of a magnetic interface. The Sobczak (U.S. Pat. No. 3,942,145), Walker (U.S. Pat. No. 3,879,602), Kikuchi et al. (U.S. Pat. No. 3,690,432) and Cherry et al. (U.S. Pat. No. 3,644,855) patents are examples of the utilization of magnetic separation to achieve tactile breakaway feel. In these prior art devices there is a steady, constant increase in spring biased resistance to key movement leading to breakaway as the magnetic interface is broken or separated by the spring force, or spring biased resistance is introduced at magnetic separation, which tends to mask the breakaway either prior to or immediately after its occurrence. The Sobczak (U.S. Pat. No. 3,942,145), Walker (U.S. Pat. No. 3,879,602) and Kikuchi et al. (U.S. Pat. No. 3,690,432) patents are examples of masking of the magnetic breakaway prior to its occurrence. This is exemplified by the Load-Time curve of FIG. 9 of the Kikuchi et al. patent, and is characteristic of those devices in which the magnetic interface is broken by kinetic energy stored in a spring (as in Sobczak, Walker and Kikuchi), as distinguished from direct operator force. The Cherry et al. (U.S. Pat. No. 3,644,855) patent is an example of masking of the breakaway immediately after its occurrence. The magnetic interface is broken by direct operator force in Cherry et al., but the magnetic breakaway is masked by the spring resistance introduced at breakaway. These prior art devices do not highlight the breakaway by both a sharp increase in resistance prior to breakaway and a sharp decrease subsequent to breakaway.
The present invention provides a sharp feel both prior and subsequent to the separation of the magnetic interface. In addition, the structure of the present invention provides reliable positive action capable of being repeated without malfunction throughout the life of the apparatus with which it is utilized.